Numerical simulation of interaction between a river and adjacent alluvium and saprolite groundwaters in the Piedmont physiographic province of western Georgia, U.S.A. suggests that bank storage in this crystalline-rock setting is mediated by elastic storativity rather than the more commonly assumed piston flow. Hydraulic head responds almost instantly to river stage increase despite relatively low hydraulic conductivity of surficial materials. Modeling suggests that aquifer hydraulic head variation is primarily a pressure response to stream stage change and that very little infiltration of river water into alluvium and saprolite occurs. Most unconfined aquifer bank storage models do not account for elastic effects and will therefore greatly underpredict the rate and magnitude of groundwater hydraulic head change and greatly overpredict surface water infiltration. Results suggest that groundwater adjacent to this and similar streams in crystalline-rock settings is not necessarily threatened by contamination from passing surface-water flood waves, even when a strong hydraulic connection between river and groundwater exists and even in cases where hydraulic gradient reverses and is temporarily directed from the river into its banks. Because this site is typical of many near-stream sites in the Piedmont and other crystalline-rock settings, these results may be widely applicable.

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